Various activities, such as for example contact sports or military operations, require the use of helmets to attempt to protect participants from injury to their heads due to impact forces that may be sustained during such activities. As helmets protect against head injury, it follows that the science of head injury should underlie the development and testing of helmets to achieve the greatest effectiveness.
Concussion science is not well understood. Holborn, in 1943 and 1945, proposed rotational acceleration as the primary concussive mechanism. Gurdjian, Lissner, and others in the 1950's and 60's however thought that deformation of the skull and pressure waves that propogated through the cranial vault were the most offending agents. In the 1970's and 80's experiments by Gennarelli, Ommaya, Thibault, Adams and others produced head injuries in monkeys and concluded that rotational acceleration injuries produced diffuse, deep brain injury and that linear acceleration produced focal, superficial damage: concussive injury was more easily produced with rotational acceleration. Recent experiments by Hardy using high-speed bi-planar x-ray imaging to track the displacement of neutral-density radio-opaque markers in the brain of cadavers during impacts have shown that all head impacts produce a figure of eight movement within the brain involving both linear and rotational components. Bayly et al used human volunteers and MRI imaging to measure brain deformation and found that angular acceleration and rotation occurs with linear acceleration forces.
Helmet testing in sport became formalized with motor vehicle racing. The British Standards Institute produced documents in 1952 and 1954 pertaining to the testing of motor vehicle helmets. They dropped a wooden block onto a helmeted headform made of horizontally laminated birch, with a moisture of 12% and that the wood be straight, without defect or “dote” (Neuman). In 1956 the Sport Car Club of America asked George Snively to investigate helmet performance. He put helmets on cadavers and subjected them to severe impacts recording the presence or absence of a skull fracture. He improved his technique and in 1969 put helmets on a 12 lb. K-1A magnesium alloy head form and measured the acceleration to impact. In 1966 the American Standards Association published standards using Snively testing techniques of impacting a mobile metal head form but suggested that there should be time limits placed upon the impact. In 1969 the National Operating Committee for Athletic Equipment (NOCSAE) published a standard for football helmets incorporating a more lifelike head form and a drop test paradigm. The helmeted head form is dropped from a prescribed distance onto an anvil and the central accelerometer measures the deceleration. Maximum values of deceleration are used for certification. These values typically range from 275-300×g (force of gravity). This has been a standard method of helmet testing ever since.
It has been recognized for some time that helmet testing has not taken rotational acceleration into account and that helmets may protect against certain types of severe injury but may not be protecting against concussive injury. Science has found that a rotational force component is present in every impact but this is has not been properly accounted for in present helmet standards and certification. Biokinetics and Associates Ltd. is an engineering firm that was employed by the National Football league for helmet testing and developed a pendulum impact test onto a mobile helmeted head form. Pellman Neurosurgery 58:78-96, 2006 reported that data from this has been used to update the NOCSAE drop test by placing the impactor and helmeted mobile head form in a horizontal plane. Results from these recent attempts have been questioned as to their reproducibility and clinical relevance.
A need exists for an improved method and apparatus for testing helmets.